Both “I am a friend of Anna’s” and “I am a friend of Anna” are grammatically and structurally correct possessive sentences.
The first construction, “I am a friend of Anna’s,” is the so-called double possessive or, in linguistic terms, the “post-genitive,” which simply means the preposition “of” is followed by a possessive-case or an absolute-possessive pronoun. In the double-possessive sentence at hand, the sense is that the speaker is one of the friends of Anna. This, you will note, has precisely the same sense as that of the second sentence, “I am a friend of Anna.”
If that’s the case, why bother using the double possessive construction at all? Why not just stick to the simpler single possessive—in this particular case the noun “Anna” without the apostrophe-“s”?
The reason is that over so many centuries of usage, native English speakers have found the double possessive useful as default usage that clearly distinguishes between possession of something as an attribute and possession as simple ownership of something. Both in the written and spoken form, the use of the apostrophe-“s” after the possessor noun makes it unnecessary for the speaker to elaborate on or clarify the intended sense of the statement.
Consider that someone has said this: “Amelia has a painting of my father.” This is an ambiguous statement that could either mean that Amelia has a painting done by the speaker’s father, or that Amelia possesses a painting she has done of the speaker’s father.
Now let’s add the apostrophe-“s” to the noun “father”: “Amelia has a painting of my father’s.” This time there’s no doubt that the speaker’s father—not Amelia—is the painter of the picture. Amelia only has that painting in her possession.
Always remember this general rule when the double possessive is used: What follows the preposition “of” in the double-possessive construction is invariably a definite human noun, not an indefinite, inanimate, or abstract noun. Following this rule, the use of the double possessive in this sentence is incorrect: “Alberto is a patron of the art gallery’s.” In this particular instance, only the single-possessive construction will be grammatically and semantically correct: “Alberto is a patron of the art gallery.”
